A DC-DC converter circuit is provided with a switching circuit which is a primary side switching element and which switches a DC power supply, with a transformer to whose primary side winding the output of said switching circuit is supplied and which outputs at its secondary side winding a voltage which has been changed by a predetermined voltage change ratio, and a secondary diode for current adjustment which is connected to the secondary side winding of said transformer. Having been rectified by the secondary diode, the output is supplied to a load after being smoothed.
However since, when it is turned off, due to accumulated carriers, the secondary diode has a time period trr in which it can conduct current (this time period is termed the reverse recovery time, or the recovery time), accordingly a through current flows in the secondary winding of the transformer during this reverse recovery time, and, due to this, there is the problem that a surge voltage is applied to the rectification element when it has been turned off.
Thus, in order to eliminate this surge voltage, a DC-DC converter circuit has been proposed in which a CR snubber circuit is connected to the secondary side winding.    patent document #1: Japanese Laid-Open Patent Publication 2008-79403;    patent document #2: Japanese Laid-Open Patent Publication 2003-189618.